The invention relates to a safety device having the characteristics of the preamble of claim 1.
A safety device of this type is German Patent Document DE 40 03 280 A. In this case, it is prevented that the vehicle be used by an unauthorized person in that either the inquiry code signal or the response code signal has only a short range and therefore becomes operative only when the user is in the direct proximity of the vehicle. In the interim, full-duplex transceivers have become known which permit the xe2x80x9coutwittingxe2x80x9d of the known safety device. If such a transceiver is situated in the direct proximity of the vehicles and another transceiver is situated in the proximity of the authorized user, an artificial extension of the range is achieved by way of the two transceivers. For the vehicle and for the authorized user, who emit the code signal with a small range, this code signal is received by the nearest transceiver and is transmitted to the other transceiver. As a result, a manipulation can be carried out even if the authorized user is far away from the vehicle. This manipulation is even possible if its distance is larger than the range of the code signal with the large range. The prerequisite is only that the transmission path of the two transceivers is correspondingly large.
It is an object of the invention to provide a safety device of the initially mentioned type by means of which an effective protection of the safety device is achieved against an intentional extension of the range.
The invention achieves this object by means of the characteristics of claim 1.
As a result of the identification of the response code signal, this response code signal receives an additional characteristic. Only if this identification of the response code signal corresponds to the identification expected in the receiver, will the response code signal become effective and lead, if applicable, to the desired function of the vehicle; this is, in the case of an access control, for example, to the opening of the vehicle.
This identification can be designed in different fashions. It will be particularly advantageous if the identification is not defined and predictable but appears accidental toward the outside. If, in particular, the identification is a function of the data content of the response code signal, it is true that the receiver can easily relate the identification to the data content of the response code signal and possibly identify the authorized user.
In contrast, a simple transceiver is not capable of transmitting the identification isochronously (that is, without any loss of time with respect to a code signal provided with such an identification), because it first has to examine the data bits with respect to the identification and must then transmit this identification together with the data bits to the other transceiver. There, it is necessary, in turn to again imprint this identification onto the data bits and to transmit it to the vehicle. It is easily recognizable that the double analysis or conversion of the identification of the individual data bits in the respective transceivers is time-consuming and leads to an increase of the transit time of the received signal.
If, in the receiver, the transit time of the response code signal is now proportioned such that it is identical with the transit time of the inquiry and response code signal in the case of an authorized vehicle user situated in the close range, by a transit limitation for the response code signal, the range manipulation can be recognized and the response code signal cannot not become effective which-occurs, possibly, in the case of such an actual extension of the transit route or as the result of the above-described apparent extension of the transit route, and arrives in a delayed manner.
Further improvements of the invention relate to individual measures for applying the identification and also aim at increasing the transit time of the signal arriving in the vehicle. They are the object of claims 3 to 6 and are explained in greater detail by means of the drawing.